Check the solder blobs on the edge of the circuit board in the head. Do they look as if the body tube is biting in to them? If not add a little solder.
Is the tailcap making contact properly - you can check this by removing the tailcap and using a conducting object to connect the negative of the battery to the body. Is it still dim? then the problem is the head.
I checkd the solder blobs on the edge of the circuit board in the head. I'm sure that they are fine.
In fact when the light was pretty new I had to add a little solder to the existing blobs to make better contact. The threads were so close to bottoming out before contact was made.
I also tried bypassing the switch as you suggested but alas to no avail.
Or you can try to power the emitter by using 2 AA batteries (the less current discharge the better, i use some made for low drain devices, commonly found in stores where they cost almost nothing) briefly in direct drive to see if the emitter is faulty if nothing else helps.
Ikea (EU thing) has 10 packs of AA for like 2-3€. All my remotes are very happy with them and most AA flashlights seems to like em.